There is a research center in Thermi (east of Thessaloniki, on the way to Halkidik)i, which is called:
Agricultural Research Center of Macedonia and Thrace (the Greek name you gave corresponds to that)
P.O. Box 60125
GR-570 01 Thermi
It belongs to the Aristotelian University in Thessaloniki, Dr. Eleni Maloupa is scientifically responsible of the Balkan Botanic Garden at Kroussia Mountains, Greece (in the Kilkis prefecture). She may be contacted at
bbgk@bbgk.gr,
+30 2310 471613
+30 2310 478907
I don't know if her reserch center produces plants for nurseries, but they do reseach on plants specifically suited for the Balkan region.
I have been in email contact with her once, but unfortunately never followed up her invitation to visit the center,